Ultrafast nonlinearity which results in modulation of linear optical response is a basis for development of time-varying media, in particular those operating in the epsilon-near-zero regime. Here, we demonstrate that the intraband excitation of hot electrons in the epsilon-near-zero media results in the strong second-harmonic modification while the changes in linear transmission are negligible. We also show that nonlinear response of anisotropic epsilon-near-zero materials can be controlled by coupling to vibrational modes and influences the polarisation of the reflected light.
We report a theoretical and experimental study of tightly focused vectorial vortex beams propagating through a strongly anisotropic epsilon-near-zero metamaterial. The longitudinal field generated upon focusing can couple to the longitudinal plasmonic resonance featured by the metamaterial with an efficiency which depends on the numerical aperture of the objective, the initial state of polarisation and the quality of the metamaterial. Theoretical predictions and experimental observations prove this interaction to be able to transform any vectorial vortex into an azimuthal beam, aside from the special case of an ideal radial beam. The latter resilience is broken with the introduction of defects in the initial state of polarisation, so that an azimuthal beam is obtained also in the case of non-ideal radial polarisation. We investigate how a change in the metamaterial quality influences the efficiency of the process, as well as its spectral dependence.
Copper presents an alternative plasmonic constituent benefiting from its high natural abundance and low-cost which makes this material very attractive for commercial exploitation. In this work, we present an inexpensive method for the fabrication of copper nanorod-based metamaterial with controllable dimensions and its intrinsic tunable optical properties determined by the geometry of the nanorod array and surrounding media. Copper nanomaterials are often at a disadvantage compared to those produced using noble metals because of their potential for oxidation. Reframing this problem, we developed a procedure for the controllable growth and reduction of copper oxide layers of nanometric thickness via electrochemical oxidation in an alkaline electrolyte at a rate of approximately 0.23 nm/min. The high refractive index sensitivity of these metamaterials enabled the complex electrochemistry of copper to be monitored via in-situ visible light spectroscopy and the subsequent correlation of the optical spectra with the oxidation and reduction processes.
Realizing ultrafast optical control of materials is imperative for advancing the field of optical information processing, nonlinear optics, and time-varying materials. Noble metal-based plasmonics has provided many platforms for achieving optical switching, using strong local field enhancement offered by plasmonic resonances and free-electron plasmonic nonlinearity. However, the switching times in such systems are traditionally constrained by the relaxation of photoexcited hot electrons. In this study, we investigate an interplay between electron relaxation lattice vibrations of the nanostructure. This is achieved by harnessing a temporal Fano-type interference between the rapid relaxation of hot electrons and vibrational dynamics within the plasmonic nanostructure. The effect provides high spectral selectivity and sensitivity to the polarisation of light and geometric parameters of the nanostructure. The results are important for development of nonlinear nanostructure with the tailored transient response.
Hot carriers generated during plasmonic decay in metal nanostructures may be utilised to improve the efficiency of photo-catalytic processes, particularly in combination with a metal oxide to drive photochemical reactions. Here, we report a novel method for the fabrication of core-shell copper/copper oxide nanorods with dimensions designed for an efficient nanocatalyst for the plasmon-driven catalytic conversion of carbon dioxide (CO2) into multi-carbon products. The initial optical properties are determined by the choice of template geometry. However, anodising the copper metamaterial in alkaline electrolytes facilitates the controllable growth of a copper oxide nanoshell. This route not only provides an additional mechanism for tuning the optical properties, but provides a designable catalytic surface over a large surface area opening the door for efficient photo-electrochemical catalysis of CO2. In this work, the fabrication techniques and optical properties.
Metamaterials provide unique opportunities for manipulation of dispersion of light waves and, therefore, polarisation and phase, as well as amplitude of transmitted and reflected waves. Here we report on using linear and nonlinear properties of nanorod and nanotube based metamaterials for shaping ultrashort optical pulses. Intensity limiters, temporal pulse shape control, as well as polarisation switching will be presented. The role on nonlocal effects in pulse propagation in metamaterials will be discussed. Using nanotube based metamaterials allows to introduce additional degree of freedom for passive and active tunability of the optical response.
Structured light finds many interesting applications in numerous fields, such as information encoding, wavefront manipulation and imaging. The opportunity to engineer the field distributions of optical beams opens up many exciting possibilities for achieving new optical interactions with materials and molecules. In this work we theoretically and experimentally investigate the interaction of cylindrical vortex beams (CVBs) with a strongly anisotropic plasmonic metamaterial, focusing on the case of radially and azimuthally polarised beams. We developed a semi-analytical model to describe the propagation of CVBs through an anisotropic slab, describing the metamaterial by means of an effective medium theory. In the tight focusing regime, the extinction properties of the metamaterial show the sample sensitivity to different symmetries of the electric field distributions, as well as the important role of the longitudinal field components of the beam on the extinction. Strong dichroism of the anisotropic metamaterial results in variations of the beam modal structure by differently influencing the three components of the field. Moreover, decomposing the beam intensity profiles in the Laguerre-Gauss mode basis reveals a non-negligible variation of the modal content of the beam, induced by the nanorod metamaterial anisotropy. Linear, radial and azimuthal polarisation states have been tested for metamaterials with different anisotropy parameters. Experimental results show a good agreement with the theoretical predictions, proving the promising potential of anisotropic metamaterials for manipulation of complex vector beams.
Plasmonic metamaterials are artificial structures whose optical response can be tailored to achieve several effects by playing with the geometrical parameters of the components. In this talk, we discuss how to apply the metamaterial design rules to develop band-stop linear filters and nonlinear filters, operating as intensity limiters. In both regimes, the filters share some common qualities: their optical response does not change for a broad range of incidence angles, at least up to 30 degrees, and is only weakly dependent on the polarisation of the incident light. These properties make these ultrathin filters useful in open field applications. The metamaterial is based on an array of gold nanotubes (i.e., a cylindrical gold shell with a dielectric core) embedded in a dielectric matrix. In the linear regime, the metamaterial displays an absorption resonance independent of the polarisation and the angle of incidence of the light, which can be tuned throughout the visible spectral range by changing the geometrical parameters of the array. In the nonlinear regime - based on free-electron Au nonlinearity and tested with ns-long pulses at 532 nm - the metamaterial limits the output peak fluence, keeping it constant across several order of magnitudes of the incoming fluence. The proposed metamaterial approach can be useful for designing optical spectral filters and intensity limiters over broad range of wavelengths.
In this work, we present a detailed experimental Raman investigation of nanostructured silicon films prepared by metalassisted chemical etching with different nanocrystal sizes and structures. Interpretation of observed one and two-phonon Raman peaks are presented. First-order Raman peak has a small redshift and broadening. This phenomenon is analyzed in the framework of the phonon confinement model. Second-order Raman peaks were found to be shifted and broadened in comparison to those in the bulk silicon. The peak shift and broadening of two-phonon Raman scattering relates to phonon confinement and disorder. A broad Raman peak between 900-1100 cm-1 corresponds to superposition of three transverse optical phonons ~2TO (X), 2TO (W) and 2TO (L). Influence of excitation wavelength on intensity redistribution of two-phonon Raman scattering components (2TO) is demonstrated and preliminary theoretical explanation of this observation is presented.
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